Isocyanatoaryl sulfochlorides are known. According to German Pat. No. 947,159, they are prepared by the phosgenation of aromatic aminosulfonic acids. This process has the disadvantage that it requires the use of high boiling solvents (such as dichlorobenzene and, in particular, nitrobenzene) and that the aminosulfonic acids used as starting material are difficult to dissolve in these solvents, even at the high reaction temperatures employed. The reaction is therefore lengthy and the yield of isocyanatoaryl sulfochlorides obtained is unsatisfactory. This applies particularly to the preparation of diisocyanatoaryl sulphochlorides, one of which was obtained in only 46% of the theoretical yield in Example 3 of the above-mentioned reference. Due to these disadvantages, the process has not achieved a position of technical importance.
It is also known to prepare simple aryl sulfochlorides by the sulfonation of aromatic compounds, e.g., with chlorosulfonic acid or mixtures of chlorosulfonic acid and sulfuryl chloride (see, e.g., Houben Weyl, 4th Edition, Volume 9, pages 563-585). It is also known to prepare sulfochlorides from free sulfonic acids or salts thereof with acid chlorides. These processes which are known from the literature have hitherto not been used or only used with negative results, for the preparation of isocyanatosulfochlorides from isocyanates. This is due to various causes.
Firstly, sulfochlorination with chlorosulfonic acid requires an excess of this reagent and therefore requires working-up to be carried out in the presence of water, which causes decomposition of the isocyanate groups. Secondly, isocyanatosulfonic acids have only become known very recently and have proved to be extremely difficult to handle and extremely reactive. Furthermore, it is known that sulfonic acid groups react in an uncontrollable manner with isocyanates at elevated temperatures with elimination of CO.sub.2. Thus, under the conventional reaction conditions, the preparation of sulfochloride is liable to be accompanied to a considerable extent by other reactions including sulfone formation and anhydride formation, as well as polymerization reactions of the isocyanate group.
Isocyanatoaryl sulfochlorides containing at least two isocyanate groups, and particularly derivatives of the commercially important diphenyl methane series would, however, be interesting substances with a considerable potential for practical application since:
(1) they should have an even lower vapor pressure than the unmodified diisocyanate; PA1 (2) they have a higher polarity than the unmodified diisocyanate; and PA1 (3) the hydrolysis products thereof (aromatic diaminosulfonic acids) are presumably non-toxic. PA1 R represents a hydrocarbon group obtained by the removal of two isocyanate groups and n hydrogen atoms from a diisocyanato diphenyl methane or a mixture of diisocyanato diphenyl methane isomers; and PA1 n represents 1 or 2. PA1 (a) from 0.2 to 2.2 mol of sulfur trioxide or chlorosulfonic acid per 250 g of polyisocyanate; and PA1 (b) from 0.3 to 22 mol of an inorganic acid chloride per 250 g of polyisocyanate;
It was therefore an object of the present invention to provide polyisocyanates or polyisocyanate mixtures of the diphenyl methane series containing chlorosulfonyl groups and a process for the preparation thereof which could be carried out on a commercial scale. It was surprisingly found that, in spite of the difficulties which were to be expected, this problem could be solved by the process according to the present invention, which is described below. Furthermore, the compounds according to the present invention were found to be valuable starting materials for the preparation of polyisocyanate polyaddition products, not only embodying the above-mentioned advantages, but also imparting improved flame resistance to the polyaddition products produced therefore.